The invention relates to multi-channel signaling, and more particularly to multi-channel signaling which includes differential signaling.
Transmission of data has been achieved in several ways. Typically the most efficient way, from a number of transmission lines involved or outputs involved, is to use a single line for each data signal. This style of signaling is commonly called single-ended signaling. Another technique, which is much faster, is to use two lines per signal and for them to be differential, i.e., using differential signals. Another technique is to use a high-speed carrier and have data, in one form or another, modulating that carrier. The technique using modulation is generally a wireless technique and has, of course, the advantage of not requiring wires. A disadvantage of it, though, is that is does require special electronics to assemble all that information and get it transmitted properly, and also on the receiving end there may be not just electronics but antennas and other space-requiring hardware involved. This is generally not practical for information transfer within a circuit board or within a product such as a computer.
The primary reason that differential signaling is significantly faster than single-ended signaling is that the majority of noise that occurs will occur on both lines and has the effect of being cancelled out. This is commonly called common mode rejection. The voltage differential between the two complementary signals provides the logic state information of the data signal. Thus, at the receiving end it is the voltage differential that is detected. Noise will affect both signals equally so the differential remains the same as that transmitted.
The disadvantages of having two wires per signal, however, are significant. In the case of an integrated circuit transmitting a differential signal, that means the integrated circuit itself has two output pins for each signal, and the pin count significantly impacts cost and reliability as well as size of the integrated circuit itself. The number of pins affects the cost of making the semiconductor wafers that have the integrated circuit die as well as the package which houses or carries the integrated circuit when shipped to the end user. The size aspect impacts the end user because the integrated circuit is generally located on a product. The available space on a printed circuit board that contains the integrated circuit in the product is typically desired to be as small as possible. It is advantageous if there is less space taken up by the integrated circuit.
By way of example, a channel of data may be 72 pins. The 72 pins represent both the true and complements of the information. If single-ended signaling were utilized instead, only 36 pins would be required. On the other hand if differential signaling is utilized, for each additional channel, seventy-two more pins are required instead of thirty-six for single-ended. Thus there is a significant disadvantage in adding an additional channel. Accordingly, there is a need for high-speed data transmission without having to add additional pins.